Perhaps it is an illusion, but there seems to be a large fin, like that of a great fish, knifing up through the deep desert sands- and it is clear as the finest pane of window glass.

Silicarcharadon (CR 3; XP 800)
N Large magical beast
Init +4; Senses darkvision 60 ft., low-light vision; Perception +6
AC 19, touch 9 , flat-footed 19
(+10 natural, -1 size)
hp 30 (4d10+8)
Fort +6, Ref +4, Will +2
DR 5/bludgeoning; Immune fire; Resist acid 5
Vulnerable sonic
Speed 20 ft., swim 60 ft. (glass, sand)
Melee bite +7 (1d8+3 plus bleed)
Space 10 ft.; Reach 5 ft.
Special Attacks bleed (1d4), merge with glass
Str 16, Dex 10, Con 14, Int 2, Wis 12, Cha 6
Base Atk +4; CMB +8; CMD 18 (cannot be tripped)
Feats Improved Initiative, Weapon Focus (bite)
Skills Perception +6, Stealth +9 (+13 in glass); Racial Modifiers +4 to Stealth (+8 in glass)
SQ no breath
Environment desert
Organization solitary, school (2-7)
Treasure none
Special Abilities
Merge with Glass (Ex) A glass shark behaves in a silicate environment (glass, sand, obsidian) as though it were water. It is effectively invisible while in those environments, though if it has fed, it is visible in glass. Even a thick pane of window-glass is enough to support a glass shark.

A bizarre life form that lives in media of sand and glass, silicarcharadons, or, simply, glass sharks are often mistaken for constructs or perhaps an illusion, but their blood-drawing bites are all too real. It is believed that they can subsist on subtle impurities in glass, and perhaps on light itself, but their digestive systems are also capable of turning flesh and blood to their use, and they do so with alacrity. As they do not respire, glass sharks can even move about on a hard surface, in the air, but they are slow out of their element, and do so only when they can ensure the capture of prey. Though invisible in water, silicarcharadons are even more unwieldy, as their propulsion fins cannot gain purchase in a silty bottom, and they typically avoid water.

It is likely that there are subspecies of glass sharks, and presented here is a typical one. Smaller glass sharks would likely be unnoticed by sentient beings, eating insects and small impurities, but larger types, like the blacksand shark, are very dangerous.

Blacksand Shark (CR 4): This huge, sleek predator is rare indeed, only dwelling in large obsidian deposits, deep ash-fields and vast black-sand beaches. A blacksand shark is a giant glass shark.

Needle Shark (CR 5): These long, thin silicarcharadons are faster (swim +10 ft.) than their relatives, and are deadly bloodletters (1d8 bleed damage per bite). Needle sharks are advanced glass sharks.

Silicarcharadons are the primary ‘animal’ inhabitants of The Glass Sea, the Glass Sands, and other significant patches of NewGod War-created glass areas. The ‘needle sharks’ of the Glass Sands are a deadly subspecies found in the Ghadab.